Cellulose derivatives, on account of their excellent properties and their physiological safety, have versatile uses, for example as thickeners, adhesives, binders, dispersants, water-retention agents, protective colloids, stabilizers and also as suspension, emulsifying and film-forming media.
A field of application for cellulose derivatives which is of particular interest here is the extrusion of construction material systems. The extrusion of construction material systems such as cementitious systems and fibrous cementitious systems is broadly used for producing slabs and profiles for walls, roofs, floors, etc. Hereinafter this procedure will be termed cement extrusion.
Cement extrusion is a process for shaping pasty cement-containing mixtures into any desired profiles by pressing through a die mouth. The elements obtained in this manner may be used in a versatile manner, in particular in construction applications. Here, firstly they can replace cement masses which have been cast hitherto; secondly, they can render accessible profile shapes which are not accessible by the conventional casting process. Examples of extruded construction elements extend from small angled profiles which have a width of only a few centimetres, up to large construction slabs which can have a width of 60 cm and in theory any desired length. Extruded cement masses comprise in principle cement as binder, possibly other binders, aggregates (sands) and/or lightweight aggregates, and also cellulose ethers, in particular methylcellulose, as water-retention agent, plasticizer and lubricant. Frequently, natural and/or synthetic fibres are also added. The water-retention agents, plasticizers and lubricants are referred to as additives hereinafter. Owing to their water-retention capacity, such additives also prevent the phase segregation between liquid and solid phases. The amount of the additives used is generally between 0.3 and 2.5%.
Due in part to the shortcomings of the art-known additives, cement extrusion is still attended by a number of disadvantages which limit its commercial use. It is known to those skilled in the art that, as additives for cement extrusion, use is made of cellulose derivatives, in particular derivatives of methylcellulose such as methyl hydroxyethylcellulose (MHEC) or methyl hydroxypropylcellulose (MHPC). However, these additives do not display the desired property profile. In particular MHPC loses its efficacy at relatively high temperatures, and other cellulose derivatives tend to lead to a high extrusion pressure and to a rough uneven surface of the extrudate.
GB-A-514,917 describes irreversibly crosslinked cellulose derivatives and a process for producing water-soluble cellulose ethers which are crosslinked with a bifunctional reagent. The purpose of GB-A-514,917 was to produce cellulose ethers which have an unusually high viscosity in water. Preferably, the products exhibit a viscosity increase of 400%.
U.S. Pat. No. 4,321,367 likewise describes a process for preparing crosslinked cellulose ethers, again with the purpose of providing products having increased viscosity in aqueous solution. Preferably, the viscosity of a 2% strength by weight solution is increased by at least 50%; in the most preferred variant, the viscosity of a 2% strength by weight solution is increased by at least 100%.
In these publications, while irreversibly crosslinked cellulose derivatives are disclosed, they are not described in ranges which would impart the desired theological properties. Also, none of these publications suggests the use of such products in cement extrusion or even describe the degrees of substitution suitable for such products.
Hence, there is still no prior art additive which solves the problems occurring in cement extrusion. There is, therefore, an urgent requirement to provide improved additives which exhibit water-retention capacity at high temperatures, lower the extrusion pressure in cement extrusion, enhance the surface quality of the extrudate, permit a high exit velocity, and achieve shape stability of the extrudate.
It was an object of the present invention to provide improved additives for the extrusion of construction materials which avoid the problems described.